Gas flow systems which require a high degree of pure gas must be sampled periodically to determine the degree of contamination of the gas. In connection with sampling gas flows for particulate contamination it is common to utilize a fine mesh filter at the exhaust end of a supply system as a trap or catcher for particulate matter that is contained in the gas flowing through the system. The filter, after a period of time, is removed and inspected visually for particulate matter after a known volume of gas flows through the system and the filter. The filter used to catch the particulate matter in the gas flow is typically quite delicate and highly restrictive to the flow of gas in comparison with the supply system. As a consequence, the gas flow rate allowed by the differential pressure across the filter is usually much lower than the operating gas flow rate for the gas supply system. Thus, the sample of particulate matter is taken while the gas flow is at a lower velocity than the operating velocity of the gas flow. A low velocity gas sample is not always representative of the actual cleanliness or contamination of the gas flow in a supply system because the upstream laminar flow in the supply system does not carry all of the particulate matter downstream to the capture filter. As a consequence, some very dirty gas systems can be sampled so delicately that the sample of particulate matter obtained by an exhaust filter inaccurately shows less contamination than the actual contamination of the gas in the system. Thus, trapping of a particulate matter in a filter in an exhaust outlet for determination of contamination is not particularly reliable.